Aluminum alloy sheet for automobile structural member use, automobile structural member, and method for producing aluminum alloy sheet for automobile structural member use

ABSTRACT

Provided are: an aluminum alloy sheet for automobile structural member use having strength, formability, crushing properties, and corrosion resistance at excellent levels with balance; an automobile structural member; and a method for producing an aluminum alloy sheet for automobile structural member use. The aluminum alloy sheet for automobile structural member use is an Al—Mg—Si aluminum alloy sheet containing, in mass percent, 0.4% to 1.0% of Mg, 0.6% to 1.2% of Si, and less than 0.7% of Cu, with the remainder consisting of Al and impurities. The aluminum alloy sheet has an earing rate of −10.0% to −3.0%.

TECHNICAL FIELD

The present invention generally relates to Al—Mg—Si (6XXX-series)aluminum alloy sheets produced through regular rolling. In particular,the present invention relates to such an aluminum alloy sheet being usedfor automobile structural members and having excellent crushingproperties.

As used herein, the term “aluminum alloy sheer” refers to a materialaluminum alloy sheet which is a rolled sheet having undergone hotrolling and/or cold rolling, where the rolled sheet is after receivingtempering such as solution treatment and/or quenching, but beforereceiving forming into an automobile structural member and artificialaging such as paint bake hardening. In the following description,aluminum is also simply referred to as “Al”.

BACKGROUND ART

Recent years have seen more and more increasing social needs for weightreduction of automobile bodies, in consideration typically of globalenvironment. To adapt to such needs, aluminum alloy materials, insteadof conventional ferrous materials such as steel sheets, are applied topanels (e.g., outer panels such as hoods, doors, and roofs, and innerpanels) and reinforcements, such as bumper reinforcements (bumper R/Fs)and door beams, of the automobile bodies.

For further weight reduction of the automobile bodies, demands are madeto apply such aluminum alloy materials also to side members and othermembers, frames, pillars, and other automobile structural members,because these members among automobile members, when reduced in weight,particularly contribute to weight reduction of automobile bodies. Theautomobile structural members have to be produced from aluminum alloymaterials that have strength and formability equivalent to those for theautomotive body panels and, in addition, have excellent impactabsorption and crushing properties (crushing resistance or crushingcharacteristics) upon body collision, for the sake of passenger safety.

A non-limiting example of methods for measuring the crushing propertiesis “VDA 238-100 Plate bending test for metallic materials” (hereinafteralso referred to as a “VDA bending test”) standardized by GermanAssociation of the Automotive Industry (VDA). The VDA bending test hasbeen employed in evaluation typically in Europe to meet the increased(stricter) level of automobile crash safety standards, and this requiresframes, pillars, and other automobile structural members that have moreexcellent crushing characteristics.

Conventionally known ways to improve the crushing properties of6XXX-series aluminum alloys for automobile structural member use includetechniques of controlling the size or shape of grains, and the areafraction of Cube orientations. For example, Patent Literature (PTL) 1discloses a 6XXX-series aluminum alloy sheet in which, of grains, thegrain size in the through-thickness direction is specified, and theratio of the grain size in the sheet rolling direction to the grain sizein the through-thickness direction is controlled.

PTL 2 proposes a 6XXX-series aluminum alloy sheet in which the amountsof Mg, Si, and Cu are controlled, and the average area fraction of Cubeorientations in a sheet cross section is controlled to be 22% or more.PTL 2, which has an objective to improve crushing properties, mentionsthat the VDA bending test as an evaluation test for sheet crushingproperties has a correlation with crushing properties upon automobilecollision. The VDA bending test gives a bending angle, which enablesquantitative evaluation for the level of crushing properties of thesample.

CITATION LIST Patent Literature

-   -   PTL 1: Japanese Unexamined Patent Application Publication (JP-A)        No. 2001-294965    -   PTL 2: JP-A No. 2017-88906

SUMMARY OF INVENTION Technical Problem

However, of such aluminum alloy sheets, the crushing properties aretrade-off against the formability, and are also trade-off against thestrength. For example, an aluminum alloy sheet, when produced by aproduction method regulated to provide better formability, has lowercrushing properties. An aluminum alloy sheet, when produced from amaterial aluminum alloy having controlled metal contents to have higherstrength, has lower crushing properties. As described above, safetystandards typically for automobiles have become more and more strict,and this demands for aluminum alloy sheets that have such properties asto give higher safety. Accordingly, demands are made to develop aluminumalloy sheets having still better crushing properties withoutdeterioration in strength and formability.

In addition, such automobile structural members require not onlystrength, formability, and crushing properties, but also corrosionresistance to corrosive environments such as salt water, from theviewpoint of reliability as structural members. Specifically, structuralmembers derived from aluminum alloys require such excellent corrosionresistance as to resist grain-boundary corrosion and other corrosionover the long term.

Under these circumstance, the present invention has an object toprovide: an aluminum alloy sheet for automobile structural member use,an automobile structural member, and a method for producing such analuminum alloy sheet for automobile structural member use, where thealuminum alloy sheet is a 6XXX-series aluminum alloy sheet producedthrough regular rolling and excels in strength, formability, crushingproperties, and corrosion resistance with balance as a material sheet.

Solution to Problem

After intensive investigations to achieve the object, the inventors ofthe present invention have found that an aluminum alloy sheet havingstrength, formability, crushing properties, and corrosion resistance atexcellent levels with balance can be obtained by appropriately adjustingthe chemical composition of the material aluminum alloy, defining theanisotropy of the aluminum alloy texture by earing rate, and controllingthe earing rate within a predetermined range.

Specifically, the present invention provides an aluminum alloy sheet forautomobile structural member use. The aluminum alloy sheet is anAl—Mg—Si aluminum alloy sheet containing, in mass percent, 0.4% to 1.0%of Mg, 0.6% to 1.2% of Si, and less than 0.7% of Cu, with the remainderconsisting of Al and impurities. The aluminum alloy sheet has an earingrate of −10.0% to −3.0%.

Ina preferred embodiment of the present invention, the aluminum alloysheet for automobile structural member use contains Mg in a content, inmass percent, of 0.4% to 0.6%. Ina preferred embodiment of the presentinvention, the aluminum alloy sheet for automobile structural member usecontains Si in a content, in mass percent, of 0.6% to 0.8%. Inapreferred embodiment of the present invention, the aluminum alloy sheetfor automobile structural member use has such bake hardenability as tohave a 0.2% yield strength of 215 MPa or more after receiving artificialaging at a temperature of 180° C. for 20 minutes.

The present invention also provides an automobile structural memberderived from any of the aluminum alloy sheets for automobile structuralmember use.

The present invention also provides a method for producing an aluminumalloy sheet for automobile structural member use. The method is a methodfor producing an Al—Mg—Si aluminum alloy sheet and includes the step ofcasting an aluminum alloy containing, in mass percent, 0.4% to 1.0% ofMg, 0.6% to 1.2% of Si, and less than 0.7% of Cu, with the remainderconsisting of Al and impurities, to give an ingot. The ingot thenreceives homogenizing, hot rolling, cold rolling, annealing,solutionizing (solution treatment), and quenching. A rolling reductionin the cold rolling step is controlled to be 40% or more; and a heattreatment temperature in the annealing step is set to be 275° C. orhigher.

Advantageous Effects of Invention

The present invention can provide an aluminum alloy sheet which is usedfor an automobile structural member and which has strength, formability,crushing properties, and corrosion resistance at excellent levels withbalance, by appropriately regulating the chemical composition of thematerial aluminum alloy and allowing the aluminum alloy to haveanisotropy in texture.

The present invention enables production of an aluminum alloy sheet forautomobile structural member use, which aluminum alloy sheet excels instrength, formability, crushing properties, and corrosion resistance,and enables production of an automobile structural member from thealuminum alloy sheet, by adjusting the chemical composition of thematerial aluminum alloy and regulating the cold rolling reduction andthe annealing heat treatment temperature in the production process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a system for a VDA bending test toevaluate crushing properties;

FIG. 2A is a front view of the punch in FIG. 1; and

FIG. 2B is a side view of the punch in FIG. 1.

DESCRIPTION OF EMBODIMENTS

Hereinafter detailed description will be made on reasons for specifyingthe chemical composition and the earing rate of an aluminum alloy sheetfor automobile structural member use, and on reasons for specifyingconditions and factors in a method for producing an aluminum alloy sheetfor automobile structural member use, where the aluminum alloy sheet andthe method are according to an embodiment of the present invention (thepresent embodiment).

The description will be made based on the premise that the Al—Mg—Si(hereinafter also referred to as “6XXX-series”) aluminum alloy sheetaccording to the embodiment of the present invention is used not forautomotive body panels as in conventional use, but for the automobilestructural member or members.

The automobile structural member (hereinafter also simply referred to as“structural member”) therefore requires to have not only formability asrequired of the conventional automotive body panels, but also excellentcrushing properties and high yield strength even after artificial aging,where these properties are properties necessary particularly for theautomobile structural members. A structural member, if lacking any ofthese properties, is insufficient as a structural member at which thepresent embodiment aims.

Accordingly, the requirements in the present embodiment as describedbelow are signified to meet and balance among these specific demandproperties to be used for the structural members.

As used in the present embodiment, the term “XX to YY” means that thefactor in question is equal to or greater than the lower limit value(XX) and equal to or less than the upper limit value (YY).

Aluminum Alloy Sheet Chemical Composition

To provide the demand properties as the structural member, the Al—Mg—Sialuminum alloy sheet according to the present embodiment contains, inits chemical composition in mass percent, 0.4% to 1.0% of Mg, 0.6% to1.2% of Si, and less than 0.7% of Cu, with the remainder consisting ofAl and impurities.

The range, significance, or permissible level of the content of eachelement in the Al—Mg—Si aluminum alloy will be described below. Allpercentages in contents are by mass.

Mg content: 0.4% to 1.0%

Magnesium (Mg) forms, together with Si, Mg₂Si and other compound phasesand precipitates upon artificial aging such as baking finish treatment.Thus, appropriate control of the Mg content allows the aluminum alloysheet to have higher strength.

An aluminum alloy sheet having a Mg content of less than 0.4% may failto have sufficient strength as a structural member.

In contrast, an aluminum alloy sheet having a Mg content of greater than1.0% may undergo formation or precipitation of coarse particles of Mg₂Siand other compound phases upon casting and uponsolutionization-quenching treatment, and the coarse particles will workas fine fracture origins. This causes the aluminum alloy sheet to havelower crushing properties. The Mg content is preferably 0.8% or less,and more preferably 0.6% or less.

The “strength of an aluminum alloy sheet” herein can be evaluated by ameasured 0.2% yield strength (MPa) of the aluminum alloy sheet (beforeartificial aging) after solution treatment and quenching.

The strength can also be evaluated by a measured 0.2% yield strength ofan aluminum alloy sheet (after artificial aging), where the aluminumalloy sheet (after artificial aging) is derived from the aluminum alloysheet (before artificial aging) through the application of a prestrainof 2% or more and artificial aging at a temperature of 180° C. for 20minutes.

The aluminum alloy sheet, when having a higher 0.2% yield strength(before artificial aging) and/or a higher 0.2% yield strength (afterartificial aging), has higher strength and better bake hardenability(BH).

Si content: 0.6% to 1.2%

Silicon (Si) forms, together with Mg, Mg₂Si and other compound phasesand precipitates upon artificial aging such as baking finish treatment.Thus, appropriate control of the Si content allows the aluminum alloysheet to have higher strength.

An aluminum alloy sheet having a Si content of less than 0.6% may hardlyprovide sufficient strength as a structural member. The Si content ispreferably 0.7% or more, and more preferably 0.8% or more.

In contrast, an aluminum alloy sheet having a Si content of greater than1.2% may undergo formation or precipitation of coarse particles of Mg₂Siand other compound phases upon casting and uponsolutionization-quenching treatment, and the coarse particles will workas fine fracture origins to cause the aluminum alloy sheet to have lowercrushing properties. The Si content is preferably 1.1% or less, morepreferably 1.0% or less, and still more preferably 0.8% or less.

Cu content: less than 0.7%

Copper (Cu), if present in a content of 0.7% or more, forms aprecipitation free zone (also called PFZ) in the vicinity of a grainboundary as associated with aging precipitation, and the zone, which isless potentially noble than the inside of the grain, selectivelydissolves in a corrosive environment and causes the aluminum alloy sheetto have lower resistance to grain-boundary corrosion (corrosionresistance).

To eliminate or minimize this, the Cu content is controlled to be lessthan 0.7%. The Cu content is preferably 0.5% or less, and morepreferably 0.3% or less. The Cu content herein has no lower limit andmay be 0%.

Other Elements

The other elements (such as elements described below) than theabove-mentioned elements are basically regarded as impurities in thepresent embodiment. The contents of these elements are described belowregarding their upper limits, which are permissible levels when beingderived from melting materials, such as scrap, to form ingots. Thecontents have no lower limits and may be 0%.

Mn: 1.0% or less, Fe: 0.5% or less, Cr: 0.3% or less, Zr: 0.2% or less,V: 0.2% or less, Ti: 0.1% or less, Zn: 0.5% or less, Ag: 0.1% or less,and Sn: 0.15% or less.

These elements, when present in contents within the ranges, do notimpair advantageous effects of the present invention, not only whencontained as unavoidable impurities, but also when added willingly.

Aluminum Alloy Sheet Thickness: 1.5 mm or More

The thickness of the Al—Mg—Si aluminum alloy sheet according to thepresent embodiment is not limited in its lower limit, but is typically1.5 mm or more, to provide strength and rigidity necessary as anautomobile structural member. The thickness is also not limited in itsupper limit, but is typically 4.0 mm or less, in consideration oflimitations in forming such as press forming and the range of suchallowable weight increase as not to adversely affect the weightreduction effect as compared with a steel sheet as a comparativematerial. Whether the material sheet having a thickness within the rangeis formed as a hot-rolled sheet or cold-rolled sheet is appropriatelyselected.

Earing Rate: −10.0% to −3.0%

The earing rate of an aluminum alloy sheet indicates texture anisotropyand shows a strong correlation particularly with the integration degreeof Cube orientations. An aluminum alloy sheet having an earing rate ofgreater than −3.0% may include Cube orientations with a low integrationdegree, fail to restrain a shear zone in bending deformation duringcrushing, and have lower crushing properties.

In contrast, an aluminum alloy sheet having an earing rate of less than−10.0% may include Cube orientations excessively highly integrated andhave a lower breaking elongation, namely, have lower formability, as aresult of concentration of strain to the Cube orientations.

Earing Rate Measuring Method

From a test sample, a disk-shaped test specimen (blank) having an outerdiameter of 66 mm is blanked, and the test specimen receives cuppingusing a 40-mm diameter punch to give a drawn cup having a cup diameterof 40 mm. The heights of ears of the drawn cup are measured, on thebasis of which the earing rate (%) can be calculated according toFormula (1) below.

In Formula (1), hX represents the height of an ear of the drawn cup; andthe index X of h represents the measurement position of the cup height(ear height), where the position is a position making an angle of X°with the sheet rolling direction of the aluminum alloy sheet.

Earing rate(%)=[{(h45+h135+h225+h315)−(h0+h90+h180+h270))}/{½(h0+h90+h180+h270+h45+h135+h225+h315)}]×100  (1)

For illustrating its significance, Formula (1) can also be expressed asFormula (2):

Earing rate (%)={((Average of heights at four points in the 45°direction relative to the bottom(the sheet rolling direction) of thecup)−(Average of heights at four points in the 0° and 90° directionsrelative to the bottom of the cup))/(Average of heights at eight pointsin the 0°,45°, and 90° directions relative to the bottom of thecup)}×100  (2)

Crushing Properties

As used herein, the term “crushing properties” refers to such propertiesthat the resulting automobile structural member resists cracking andcrushing and, even when undergoing cracking and/or crushing, deforms tothe last in the early stages of and during deformation upon applicationof an impulsive load such as in automobile collision. A member havinggood crushing properties resists cracking and crushing and, even whenundergoing cracking and/or crushing, undergoes bending deformation likean accordion.

As described above, the crushing properties may deteriorate when thematerial aluminum alloy has a Mg content and a Si content each greaterthan the upper limit of the range specified in the present embodiment.The crushing properties can be evaluated by the VDA bending testmentioned later. Regarding the crushing properties, the aluminum alloysheet has a bending angle of preferably 93° or more, more preferably100° or more, further more preferably 105° or more, and still morepreferably 110° or more.

In the present embodiment, an aluminum alloy sheet having such crushingproperties as to give a bending angle of 93° or more is evaluated asbeing accepted for automobile structural member use. In contrast, analuminum alloy sheet having such crushing properties as to give abending angle of less than 93° is evaluated to be insufficient forautomobile structural member use.

A bending test to evaluate the crushing properties operates inaccordance with the VDA bending test prescribed in German Association ofthe Automotive Industry (VDA).

FIG. 1 illustrates, as a perspective view, how to preform the bendingtest. FIG. 2A and FIG. 2B are a front view and a side view,respectively, of a punch 3, which is a sheet-like bending jig.

Initially, two rollers 2 are disposed in parallel to each other with aroller gap L, and a sheet-like test specimen 1 is placed horizontally onthe two rollers 2 equally in length on both sides, as indicated by thedoted lines in FIG. 1.

Next, the punch 3, which is a sheet-like bending jig, is placedvertically above the sheet-like test specimen 1. Specifically, therollers 2, the test specimen 1, and the punch 3 stand so that the topedge side of the punch 3 lies at the center of the roller gap L, and thesheet rolling direction of the sheet-like test specimen 1 intersectswith the extending direction of the sheet-like punch 3 at right angles.

The punch 3 then presses from above against the central part of thesheet-like test specimen 1 to put a load F on the test specimen, therebybends the sheet-like test specimen 1 by pushing (by pressing) toward thenarrow roller gap L, and presses the central part of the bent, deformedsheet-like test specimen 1 into the narrow roller gap.

In this process, a bending angle (in degree) at the time when the loadF, which is put by the punch 3 from above, becomes maximum is measured,and the crushing properties are evaluated by the magnitude of thebending angle, where the bending angle is defined as the outer bendingangle of the central part of the sheet-like test specimen 1. It can bedetermined that, with an increasing bending angle, the sheet-like testspecimen more remains in a bending deformation state without crushingduring bending, and offers higher crushing properties.

The VDA bending test operates under conditions as follows. Thesheet-like test specimen 1 has a thickness of 2.0 mm and has a squareshape of a width b of 60 mm by a length l of 60 nm. The two rollers 2each have a diameter D of 30 mm and lie with a roller gap L of 4.0 mm,which is 2.0 times as much as the thickness of the sheet-like testspecimen 1. At the time when the load F becomes maximum, the centralpart of the sheet-like test specimen presses into the roller gap at adepth S.

As illustrated in FIG. 2B, the punch 3 has a length of a side in contactwith the test specimen 1 of 90 mm, and has a tapered narrow shape sothat its lower side (apex) has a radius r of 0.2 mm, where the lowerside presses against the central part of the sheet-like test specimen 1,as illustrated in the front view of the punch 3.

The punch 3 has two recesses each having a width of 9 mm and a depth of12 mm, in the opposite side to the apex. The punch 3 is configured toapply a load on the test specimen 1 when the recesses of the punch 3 areengaged to a loading device (not shown).

Strength

The aluminum alloy sheet according to the present embodiment, whenreceiving a prestrain of 2% or more and undergoes artificial aging at atemperature of 180° C. for 20 minutes after receiving solution treatment(solutionization) and quenching, preferably has a 0.2% yield strength(bake hardenability or BH) of 215 MPa or more.

The aluminum alloy sheet, when having a 0.2% yield strength of 215 MPaor more, can surely have strength necessary as an alloy sheet forautomobile structural member use. The 0.2% yield strength can becontrolled not only by the contents of elements in the aluminum alloy,but also particularly by the thermal hysteresis and the rollingreduction in in the steps in the after-mentioned production method.

Formability

An aluminum alloy sheet according to the present embodiment, if producedby a method with a rolling reduction in cold rolling lower than thelower limit of the range specified in the present embodiment, may havelower formability, as described later. The formability can be evaluatedby breaking elongation as indicated in working examples below, and thealuminum alloy sheet preferably has such formability as to give abreaking elongation of 25% or more.

In the present embodiment, an aluminum alloy sheet having suchformability as to give a breaking elongation of 25% or more is evaluatedas being accepted for automobile structural member use. In contrast, analuminum alloy sheet having such formability as to give a breakingelongation of less than 25% is evaluated as being insufficient forautomobile structural member use.

Method for Producing Aluminum Alloy Sheet for Automobile StructuralMember Use

Next, a method for producing the aluminum alloy sheet according to thepresent embodiment will be illustrated below.

The method for producing an aluminum alloy sheet for automobilestructural member use according to the present embodiment is a methodfor producing an Al—Mg—Si aluminum alloy sheet. The method includes thesteps of casting, homogenizing, hot rolling, cold rolling, annealing,solution-treating, and quenching, where the step of casting preforms onan aluminum alloy having the chemical composition. In the method, arolling reduction in the cold rolling step is controlled to be 40% ormore, and a heat treatment temperature in the annealing step is set tobe 275° C. or higher.

The earing rate as specified in the present embodiment can be obtainedby appropriately regulating, among these production steps, the rollingreduction in the cold rolling and the temperature in the annealingprocess within the numerical ranges. Each step or process will beillustrated in detail below.

Melting and Casting

Initially, in the melting-casting step, a molten aluminum alloy isprepared by melting to have a chemical composition within the range ofthe 6XXX-series chemical composition and is cast. The casting operatesby a casting technique appropriately selected from regularmelting-casting techniques such as continuous casting and semicontinuouscasting (direct chill casting; DC casting).

Homogenization

Next, the cast aluminum alloy ingot receives homogenization prior to hotrolling. The homogenization (soaking) is significant not only forhomogenization of the microstructure (elimination or minimization ofsegregation in grains in the ingot microstructure), which is a commonobjective, but also for sufficient solid-solution (solutionization) ofSi and Mg. The homogenization may work under any conditions withoutlimitation, as long as the homogenization can achieve the objects. Thehomogenization (soaking) may operate as common single or single-stagetreatment.

The homogenization preferably operates at a temperature for a (holding)time appropriately selected within the ranges respectively from 500° C.to 560° C. and one hour or longer. The homogenization, if operating atan excessively low temperature, may fail to sufficiently eliminatesegregation in grains, and the residual segregation will work as afracture origin, and this may cause the aluminum alloy sheet to havelower crushing properties.

Hot Rolling

The hot rolling of the ingot after homogenization includes a roughrolling substep of the ingot (slab) and a finish rolling substep, inaccordance with the target thickness after rolling. The hot roughrolling substep and hot finish rolling substep may employ any of rollingmills such as reverse mills and tandem mills as appropriate.

Rough Rolling Substep

Hot rolling in the hot rough rolling substep, if started at a starttemperature higher than the solidus temperature, may cause burning, andthis may impede the hot rolling itself. Hot rolling, if started at astart temperature lower than 350° C., may require an excessively highload to operate on any material after the homogenization step, and thismay impede the hot rolling itself. To eliminate or minimize these, hotrolling operates at a hot rolling start temperature selected within therange from 350° C. to the solidus temperature, to give a hot-rolledsheet having a thickness of about 2 to about 8 mm. Annealing (heattreatment) of this hot-rolled sheet before cold rolling is not alwaysnecessary, but may be performed.

Hot Finish Rolling

After the hot rough rolling, hot finish rolling preferably operates toan end temperature from 250° C. to 350° C. The hot finish rolling, ifoperating to an excessively low end temperature lower than 250° C., maycause lower productivity due to a higher rolling load. In contrast, thehot finish rolling, if operating to an excessively high end temperaturehigher than 350° C. so as to give a recrystallized microstructurewithout a large amount of residual deformed microstructure, may highlypossibly cause the aluminum alloy sheet to have lower crushingproperties, due to precipitation of coarse Mg₂Si particles.

Annealing (heat treatment) of this hot-rolled sheet before cold rollingis not always necessary, but may be performed.

Cold Rolling

The step of cold rolling the hot rolled sheet to a desired thickness,when operating at a high cold rolling reduction, can introduce strainhomogeneous in the through-thickness direction, and this will givehomogeneous, fine, equiaxial grains upon the solution heat treatment.Specifically, cold rolling operating at a cold rolling reduction of 40%or more allows the texture to have such anisotropy as to give crushingproperties and formability with balance. This can give an aluminum alloysheet having an earing rate of −10.0% or more.

In contrast, cold rolling, if operating at a cold rolling reduction ofless than 40%, may introduce little strain, and this may cause thedeformed microstructure after hot rolling to remain. This may cause thealuminum alloy sheet to have an earing rate of less than −10.0%. Theresulting aluminum alloy sheet may have better crushing properties, butsignificantly lower formability. To eliminate or minimize this, the coldrolling operates at a cold rolling reduction of 40% or more.

The cold rolling reduction is preferably 60% or more.

Annealing Process

An annealing process at a temperature of 275° C. or higher allows thenuclei of Cube orientations remained after cold rolling topreferentially grow without coarsening and gives an aluminum alloy sheethaving an earing rate of −3.0% or less. As a result, the aluminum alloysheet can have not only excellent formability as with conventionalequivalents, but also satisfactory crushing properties. Annealing, ifoperating at a temperature of lower than 275° C., which is equal to orlower than the recrystallization temperature, may fail to inviterecrystallization, to cause the aluminum alloy sheet to have an earingrate of greater than −3.0%, and to thereby have significantly lowercrushing properties although having good formability.

The annealing temperature is preferably 300° C. or higher.

The annealing process preferably operates at a rate of temperature riseof 1 to 500° C./hr. The annealing process, if operating at a rate oftemperature rise of less than PC/hr, may cause grains to coarsen andhave larger sizes, and this may cause the aluminum alloy sheet to tendto have lower crushing properties. The annealing process, if operatingat a rate of temperature rise of greater than 500° C./hr, may cause Cubeorientation nuclei to be present in a small number, and this may causethe Cube orientations after solution treatment to be present in asmaller area fraction and cause the aluminum alloy sheet to tend to havelower crushing properties.

Solution Treatment and Quenching

After the cold rolling, the workpiece receives a solution treatment, andsubsequent quenching down to room temperature. The solution treatmentand quenching may preform using a common continuous heat treatment line.However, for sufficient amounts of solid-solutions of the elements suchas Mg and Si, it is preferred that the workpiece receives the solutiontreatment at a temperature from 500° C. to the melting temperature, andthen receives quenching down to room temperature at an average coolingrate of 20° C./sec or more. The workpiece, if receiving a solutiontreatment at a temperature lower than 500° C., may include smalleramounts of solute Mg and solute Si, because Mg—Si compounds and othercompounds formed before the solution treatment undergore-solutionization insufficiently.

The workpiece, if receiving quenching at an average cooling rate of lessthan 20° C./sec, may highly possibly fail to include sufficient amountsof solute Si and solute Mg, because Mg—Si precipitates predominantlyform during cooling, and this lowers the amounts of solute Mg and soluteSi. To surely provide a cooling rate within the range, the quenchingoperates with a selective means under selective conditions, wherenon-limiting examples of the means include air cooling means such asfans; and water cooling means such as mist, spraying, and immersion.After the solution treatment as above, the workpiece may receivepre-aging as appropriate.

Automobile Structural Member

The present embodiment also relates to an automobile structural membermade from the aluminum alloy sheet. The aluminum alloy sheet accordingto the present embodiment has strength, formability, and crushingproperties at excellent levels with balance as a material sheet, and cangive an automobile structural member having still better safety.

The present invention will be illustrated in further detail withreference to several examples below. It should be noted, however, thatthe examples are by no means intended to limit the scope of theinvention that various changes and modifications can naturally be madetherein as appropriate without deviating from the spirit and scope ofthe invention as described herein; and that all such changes andmodifications should be considered to be within the scope of theinvention.

Examples

Ingots of 6XXX-series aluminum alloys having the chemical compositionsgiven in Table 1 were prepared, from which aluminum alloy sheets forautomobile structural member use were produced under various productionconditions given in Table 2, and received earing rate measurement.

The produced aluminum alloy sheets were evaluated for strength,formability, and crushing properties by measurements of the 0.2% yieldstrength (MPa) before and after artificial aging, the breakingelongation (%), and the VDA bending angle (degree) after artificialaging. The results are also presented in Table 2.

Aluminum Alloy Sheet Production

Initially, production conditions will be described in detail. Aluminumalloys having the chemical compositions given in Table 1 were molten andcast to give ingots. The ingots received a homogenization treatment byholding the same at a temperature of 540° C. for 4 hours. The ingotsthen received hot rolling to an end temperature of 250° C. to 350° C.The workpieces further received cold rolling at the rolling reductionsgiven in Table 2 to a final thickness of 2.0 mm, and yielded cold-rolledsheets.

The cold-rolled sheets received an annealing process in an air furnace,in which each workpiece was raised in temperature at a rate of 30°C./hr, held at the annealing temperature given in Table 2 for 4 hours,and cooled down at a rate of 40° C./hr. However, the workpiece accordingto Comparative Example 1 did not receive the annealing process.

The workpieces then received a tempering treatment (T4 treatment) undercommon conditions in a heat treatment facility. Specifically, each sheetafter the annealing was heated at an average heating rate up to thesolution treatment temperature of 5° C./sec, held at a temperature of525° C. for 28 seconds for the solution treatment, and then cooled downto room temperature by fan air cooling at an average cooling rate of 20°C./sec. Immediately after the cooling, the workpieces received pre-agingby holding the same at 80° C. for 5 hours, and then received slowcooling (natural cooling) and yielded a series of aluminum alloy sheets(T4 sheets).

Earing Rate Measurement

From the produced aluminum alloy sheets, test samples were sampled, andreceived earing rate measurement by the following method. A disk-shapedtest specimen having an outer diameter of 66 mm was blanked from eachtest sample, and the test specimen received cupping using a punch havinga diameter of 40 mm and yielded a drawn cup having a cup diameter of 40mm. The ear heights of the drawn cup were measured, from which theearing rate (%) was calculated according to Formula (1).

Strength Evaluation: 0.2% Yield Strength Measurement

From each test sample, a JIS 13A tensile test specimen (20 mm by 80 mmgauge length by 2.0 mm) was sampled, and received a tensile test at roomtemperature under the following conditions, to measure the 0.2% yieldstrength. Initially, two sets of test samples after pre-aging wereprepared, and one of the two was subjected to a 0.2% yield strengthmeasurement without additional heat treatment. The other set received aprestrain of 2% or more, and underwent artificial aging at a temperatureof 180° C. for 20 minutes and subsequent measurement of 0.2% yieldstrength.

The tensile test operated so that the tensile direction be perpendicularto the sheet rolling direction.

The tensile speed was set at 5 mm/min before the load reached the 0.2%yield strength, and set at 20 mm/min on or above the 0.2% yieldstrength. The measurement operated five times, the five measurementswere averaged, and the average was defined as the 0.2% yield strength. Asample having a 0.2% yield strength of 215 MPa or more after artificialaging was determined as having sufficient strength for automobilestructural member use and evaluated as accepted.

Formability Evaluation: Breaking Elongation Measurement

From each test sample, a JIS 13A tensile test specimen (20 mm by 80 mmgauge length by 2.0 nm) was sampled, and received a tensile test at roomtemperature under the following conditions. In the tensile test using atensile tester, the test specimen was pulled at a speed of 5 mm/min, andthe elongation at the time when the test specimen broken (ruptured) wasmeasured.

The test specimen was pulled in three directions, i.e., 0° direction,45° direction, and 90° direction relative to the sheet rollingdirection, and received the measurement five times, from which fivebreaking elongations were calculated according to Formula (3) below andaveraged, and the average was defined as the breaking elongation of thesample. In Formula (3), Lo represents the gauge length before thetensile test and L represents the gauge length at break.

Breaking elongation (%)=100×(L−Lo)/Lo  (3)

A sample having a breaking elongation of 25% or more was determined ashaving sufficient formability for automobile structural member use andevaluated as accepted.

Crushing Properties Evaluation: VDA Bending Angle Measurement

The test sample after the preliminary treatment received a prestrain of2% or more and underwent artificial aging at a temperature of 180° C.for 20 minutes. From the resulting sample, a square test specimen havinga thickness of 2.0 mm, a width b of 60 mm, and a length l of 60 mm wassampled, and received a VDA bending test to evaluate crushingproperties.

The VDA bending test operated in conformity to VDA 238-100 as athree-point bending test so that the bend line be in parallel with thesheet rolling direction. In the test, the bending speed was set at 10mm/min until the load reached 30 N, and set at 20 mm/min thereafter. Thesystem was set so that bending was stopped at the time when crackingoccurred or the load decreased from the maximum load by 60 N due toreduction in sheet thickness.

The bending test operated on three test specimens per sample, the threemeasurements were averaged, and the average was defined as the bendingangle (degree) of the sample.

A sample having a bending angle of 93° or more was determined as havingsufficient crushing properties for automobile structural member use andevaluated as accepted.

The evaluation results for the strength, formability, and crushingproperties are presented in Table 2. In Table 2, a component content, aproduction condition for the aluminum alloy sheet, and a materialmicrostructure each of which does not meet or fall within the rangespecified in the present invention are underlined.

Likewise, an evaluation result for the strength, formability, andcrushing properties which result was not evaluated as accepted forautomobile structural member use is underlined.

TABLE 1 Chemical composition (mass percent, the remainder consisting ofAlloy Al and unavoidable impurities) number Mg Si Cu  1 0.4 1.0 0.2  20.4 0.4 0.2  3 0.4 0.6 0.2  4 0.4 0.8 0.2  5 0.4 1.2 0.2  6 0.4 1.4 0.2 7 0.2 1.0 0.2  8 0.6 1.0 0.2  9 1.0 1.0 0.2 10 1.2 1.0 0.2 11 0.4 1.00.7

TABLE 2 Aluminum alloy sheet Evaluation results Content of elementproduction conditions Strength (the remainder consisting of Al RollingMaterial 0.2% Yield 0.2% Yield Crushing and unavoidable impurities)reduction Annealing texture strength strength Formability properties MgSi Cu in cold temper- Earing before arti- after arti- Breaking VDAbending Alloy (mass (mass (mass rolling ature rate ficial aging ficialaging elongation angle Category number percent) percent) percent) (%) (°C.) (%) (Mpa) (Mpa) (%) (° ) Example 1 1 0.4 1.0 0.2 60 300 −5.2 99 21825  99 Example 2 1 0.4 1.0 0.2 60 350 −4.7 99 217 26 104 Example 3 1 0.41.0 0.2 60 400 −5.0 98 215 26 105 Example 4 1 0.4 1.0 0.2 80 350 −3.6100 215 29  99 Example 5 3 0.4 0.6 0.2 60 350 −5.0 101 216 26 112Example 6 4 0.4 0.8 0.2 60 350  4.9 104 220 26 106 Example 7 5 0.4 1.20.2 60 350 −4.8 110 229 27  99 Example 8 8 0.6 1.0 0.2 60 350 −4.9 100219 27 101 Example 9 9 1.0 1.0 0.2 60 350 −5.1 101 222 27  96 Com. Ex. 11 0.4 1.0 0.2 60

−1.9 101 220 27  83 Com. Ex. 2 1 0.4 1.0 0.2 60 200 −2.3 100 219 27  80Com. Ex. 3 1 0.4 1.0 0.2 60 250 −2.3 103 221 27  87 Com. Ex. 4 1 0.4 1.00.2  0 350 −12.3  101 216 16 139 Com. Ex. 5 1 0.4 1.0 0.2 10 350 −12.0 102 215 17 140 Com. Ex. 6 1 0.4 1.0 0.2 30 350 −11.6  100 217 20 125Com. Ex. 7 2 0.4 0.4 0.2 60 350 −5.8 95 208 25 116 Com. Ex. 8 6 0.4 1.40.2 60 350 −4.4 121 235 28  89 Com. Ex. 9 7 0.2 1.0 0.2 60 350 −5.6 95207 26 109 Com. Ex. 10 10 1.2 1.0 0.2 60 350 −4.5 110 234 27  90

As apparent from Table 2, Examples 1 to 9 were produced from aluminumalloys having chemical compositions within the range specified in thepresent invention, where the production operated under conditionsspecified in the present invention.

Specifically, Examples 1 to 9 are samples produced from aluminum alloyshaving chemical compositions containing, in mass percent, 0.4% to 1.0%of Mg and 0.6% to 1.2% of Si, and had an earing rate of −10.0% to −3.0%.Thus, these samples were aluminum alloy sheets having strength,formability, and crushing properties at excellent levels with balance.

Comparisons are made among Example 2 (Mg content: 0.4%), Example 8 (Mgcontent: 0.6%), and Example 9 (Mg content: 1.0%), whose conditions arecommon except for the Mg content. The comparisons demonstrate that thesamples having a Mg content from 0.4% to 0.6% have particularlyexcellent crushing properties.

Also, comparisons are made among Example 5 (Si content: 0.6%), Example 6(Si content: 0.8%), Example 2 (Si content: 1.0%), and Example 7 (Sicontent: 1.2%), whose conditions are common except for the Si content.The comparisons demonstrate that the samples having a Si content from0.6% to 0.8% have particularly excellent crushing properties.

In contrast, Comparative Examples 1 to 10 are samples produced usingaluminum alloys having chemical compositions out of the range specifiedin the present invention, or samples produced through cold rolling at arolling reduction out of the range specified in the present invention orthrough annealing at a temperature out of the range specified in thepresent invention, although the samples are produced from aluminumalloys having chemical compositions within the range specified in thepresent invention. As a result, these samples are inferior in 0.2% yieldstrength after artificial aging or in crushing properties.

More specifically, Comparative Example 1 did not receive the annealingprocess, thereby had an earing rate out of the range specified in thepresent invention, and had poor crushing properties. ComparativeExamples 2 and 3 received annealing at a temperature lower than therange specified in the present invention, thereby had an earing rate outof the range specified in the present invention, and had poor crushingproperties.

Comparative Examples 4 to 6 received cold rolling at a rolling reductionless than the range specified in the present invention, thereby had anearing rate out of the range specified in the present invention, and hadpoor formability.

Comparative Example 7 was produced from an aluminum alloy having a Sicontent less than the range specified in the present invention, andthereby had low strength.

Comparative Example 8 was produced from an aluminum alloy having a Sicontent greater than the range specified in the present invention andhad poor crushing properties.

Comparative Example 9 was produced from an aluminum alloy having a Mgcontent less than the range specified in the present invention and hadlow strength.

Comparative Example 10 was produced from an aluminum alloy having a Mgcontent greater than the range specified in the present invention andhad poor crushing properties.

Next, ingots of the aluminum alloys of alloy numbers 1 and 11 in Table 1were prepared, from which aluminum alloy sheets for automobilestructural member use were produced under production conditions asdescribed in the Aluminum Alloy Sheet Production, except for the rollingreduction in cold rolling and the annealing temperature as given inTable 3. The aluminum alloy sheets were then evaluated for resistance tograin-boundary corrosion (corrosion resistance).

Grain-Boundary Corrosion Resistance Evaluation

The resistance to grain-boundary corrosion was evaluated by a test inconformity to ISO 11846, Method B. The test samples used herein weresheet test samples after the solution treatment. To remove a surfacefilm, the test samples were immersed in a 5% NaOH (60° C.) for oneminute, rinsed, immersed in 70% HNO₃ for one minute, rinsed again, anddried at room temperature.

An aqueous solution containing HCl and NaCl (containing 30 g/L of NaCland 10±1 mL/L of 36% concentrated hydrochloric acid) was prepared as anetchant, and the test samples were immersed in the etchant at 25° C. for24 hours, where the etchant was present in an amount of 5 ml per 1 cm²of the surface area of the material. Next, the test samples wereimmersed in 70% HNO₃ and brushed with a plastic brush to removecorrosion products, rinsed, and dried at room temperature.

Next, according to the focal depth method, portions (each 30 mm by 50mm) which were determined to have deep corrosion were selected at threearbitrary points in each test sample, and the three portions wereembedded and ground to give cross sections. The depth of deepestgrain-boundary corrosion in each cross section was measured using anoptical microscope. In this testing example, samples having a maximumgrain-boundary corrosion depth of 300 μm or less were evaluated asaccepted.

The evaluation results for resistance to grain-boundary corrosion arepresented in Table 3. In the element contents in Table 3, data out ofthe range specified in the present invention are underlined.

Likewise, an evaluation result for the resistance to grain-boundarycorrosion not evaluated as accepted is also underlined.

TABLE 3 Content of element Aluminum alloy sheet (the remainderconsisting of Al production conditions Grain-boundary and unavoidableimpurities) Rolling Annealing corrosion Mg Si Cu reduction in temper-resistance Alloy (mass (mass (mass cold roiling ature Maximum depthCategory number percent) percent) percent) (%) (° C.) (μm) Example 1 10.4 1.0 0.2 60 300 259 Com. Ex. 11 11 0.4 1.0 0.7 305

As indicated in Table 3, Example 1 was produced from an aluminum alloyhaving a Cu content within the range specified in the present inventionand had excellent resistance to grain-boundary corrosion.

In contrast, Comparative Example 11 was produced from an aluminum alloyhaving a Cu content greater than the range specified in the presentinvention and had poor resistance to grain-boundary corrosion.

The results of the examples and the comparative examples demonstratethat aluminum alloy sheets meeting all the conditions for the chemicalcomposition and for the microstructure specified in the presentinvention are advantageously usable for automobile structural members.

Various embodiments have been described above with reference to theattached drawings. It is naturally understood, however, that theseembodiments are not construed to limit the scope of the presentinvention. It is apparent that a person skilled in the art could haveconceived various variations and modifications within the spirit andscope of the invention as claimed in the appended claims, and it isnaturally understood that all such variations and modifications shouldbe considered to be within the scope of the invention. Any components orconstituents in the embodiments may optionally be combined withoutdeparting from the spirit and scope of the invention.

This application claims priority to Japanese Patent Application No.2018-070252, filed on Mar. 30, 2018, the entire contents of whichapplication are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present invention allows a 6XXX-series aluminum alloy sheet producedthrough regular rolling to combine excellent crushing properties andstrength, which are properties specific to automobile structural memberuse, with formability and corrosion resistance. This enlarges theapplicability of such 6XXX-series aluminum alloy sheets as automobilestructural members.

REFERENCE SIGNS LIST

-   -   1 sheet-like test specimen    -   2 roller    -   3 punch

1. An aluminum alloy sheet, the aluminum alloy sheet being an Al—Mg—Sialuminum alloy sheet comprising, in mass percent: 0.4% to 1.0% of Mg;0.6% to 1, 2% of Si; and less than 0.7% of Cu, wherein the aluminumalloy sheet has an earing rate of −10.0% to −3.0%.
 2. The aluminum alloysheet of claim 1, wherein the aluminum alloy sheet comprises, in masspercent, 0.4% to 0.6% of Mg.
 3. The aluminum alloy sheet of claim 1,wherein the aluminum alloy sheet comprises, in mass percent, 0.6% to0.8% of Si.
 4. The aluminum alloy sheet of claim 1, wherein the aluminumalloy sheet has a bake hardenability so as to have a 0.2% yield strengthof 215 MPa or more after being subjected to artificial aging at atemperature of 180° C. for 20 minutes.
 5. An automobile structuralmember, derived from the aluminum alloy sheet of claim
 1. 6. A methodfor producing an aluminum alloy sheet, the aluminum alloy sheet being anAl—Mg—Si aluminum alloy sheet, the method comprising: casting;homogenizing; hot rolling; cold rolling; annealing; solutionizing; andquenching, the casting being performed on an aluminum alloy comprising,in mass percent: 0.4% to 1.0% of Mg; 0.6% to 1.2% of Si; and less than0.7% of Cu, wherein the cold rolling is performed to a cold rollingreduction of 40% or more, and wherein the annealing is performed at aheat treatment temperature of 275° C. or higher.